<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    * {
        padding: 0;
        margin: 0;
    }

    .max-box {
        width: 1200px;
        margin: auto;
        border: 1px solid #000;
    }

    .box {
        width: 30rem;
        height: 400px;
        border: 1px solid #000;
    }

    /* 屏幕小于750px */
    @media screen and (max-width:750px) {
        .min {
            display:flex;
            flex-direction: column;
        }

        .min div{
           width: 100%;
           height: 50px;
           background-color: red;
           margin: 20px 0 ;
        }
    }

    /* 屏幕大于750px */

    @media screen and (min-width:750px) {
        .min {
            display:flex;
            flex-wrap: wrap;
        }

        .min div{
           width: 38%;
           height: 150px;
           background-color: red;
           margin: 20px;
        }
    }

</style>

<body>
    <div class="max-box">
        <div class="box min">
            <div></div>
            <div></div>
            <div></div>
            <div></div>
        </div>
    </div>

</body>
<script>

</script>

</html>